Cyclic carbonates are important raw materials for engineering plastics like polycarbonates. They are also known for their application as polar aprotic solvents, electrolytes in secondary batteries, octane booster, and, in general, intermediates in organic synthesis Plastics of aromatic polycarbonates are widely used in electric and electronic industry, building industry, optical data storage media, automotive industry, package industry, headlamp diffuser lense and bottles for water and milk. Polycarbonates of aliphatic type are used as plasticizers, stabilizers for vinyl chloride polymers, co-monomers in polyurethane synthesis, lubricants, elastomers (functionalized PC with pendent vinyl group) and biodegradable and biomedical materials for drug delivery. Aromatic polycarbonates, for example, bisphenol-A-carbonates, are commercially manufactured by condensation of 4-hydroxydiphenylbutane and phosgene (COCl2) in the presence of substituted amines and alkali (Encyclopedia of Chemical Processing and Design, Vol 40, Ed. by J. J. McKetta and W. A. Cunningham, Marcel Dekker Inc., New York, 1992, p. 136 and Ulmann's encyclopedia of Industrial Chemistry, Vol. A 21, Ed. by B. Elvers, S. Hawkins and G. Schulz, 5th ed. VCH Verlagsgesellschaft, mbH, Germany 1992, p. 207). This method of preparation employing phosgene is highly toxic and hazardous. Preparation of polycarbonates from cyclic carbonates is an alternative attractive route.
Cyclic carbonates can be synthesized through a benign route by insertion of CO2 into the oxirane ring of epoxides. This is an efficient route for the utilization of CO2, a “greenhouse gas”, in chemicals synthesis, as an alternative to phosgene synthetic route. This reaction is catalyzed by a variety of metal catalysts from simple alkali salts and quaternary ammonium and phosphonium salts to classical organometallic complexes to different extents. Porphyrin (F. Kijima et al., J. Am. Chem. Soc. Vol. 108 (year 1986) page 391; T. Aida et al, Macromolecules Vol. 15 (year 1982) page 682 and vol. 19 (year 1986) page 8), phthalocyanine (Ji et al., Appl. Catal. A: General Vol. 203 (year 2000) page 329) and Schiff base (J. Am. Chem. Soc. Vol. 123 (year 2001) page 11498) complexes are some of those homogeneous catalysts reported to catalyze this cycloaddition reaction. Unfortunately, the metal complex catalysts that were found useful were toxic, water and air-sensitive causing handling problems and in addition required high temperature and pressure for good conversion and selectivity. In some cases high concentration of the catalyst (≧1 mol %) is required. Moreover, the processes involving these homogenous metal catalysts require additional expenses for catalyst separation and product purification.
A large number of patents have been granted towards preparing cyclic carbonate utilizing CO2 and a variety of catalyst systems. For example, U.S. Pat. No. 4,824,969 (Exxon Research & Engineering Co.) reports a process for cyclic carbonate esters from olefins in a single reaction mixture using osmium compound, copper containing co-catalyst I (e.g., CuBr2), co-catalyst II (e.g., pyridine) and water. U.S. Pat. No. 6,407,264 teaches a process involving the reaction of alkylene oxide with carbon dioxide in the presence of a catalyst system comprising of a metal halide and pyridine or pyridine derivative. U.S. Pat. Nos. 6,399,536, 5,391,767 and 6,288,202 and UK Pat Appl. GB 2352449 A1, PCT Int. Appl. WO 2000008088 A1, Ger. Offen. DE 19737547 A1 and Eur. Pat. Appl. EP 864361 A2 are all related to this process. U.S. Pat. No. 6,469,193 reports the preparation of aliphatic carbonates from aliphatic alcohols, alkyl halides and carbon dioxide in the presence of cesium carbonate and tetrabutyl ammonium iodide.
There are a few reports on the use of solid catalysts like silica supported guanidine (Barbarini et al Tetrahedron Lett. Vol. 44 (year 2003) page 2931) and MCM-supported phthalocyanine (Lu et al., J. Mol. Catal. A: Chemical Vol. 186 (Year 2002) page 33) for this reaction, however larger amounts catalyst and long reaction times (>15 h) are needed for high yield of cyclic carbonate. The Mg/Al oxide-based catalyst system reported earlier [K. Yamaguchi J. Am. Chem. Soc. Vol. 121 (Year 1999) page 4526], required a high catalyst loading of 1.8 g per g of substrate and, in addition, a substantial amount of solvent (85% v/v DMF) and longer reaction times (24 h).
Commercial production of cyclic carbonates by this non-phosgene route using quaternary ammonium salt-based catalysts has been announced recently by BASF (Filtration Industry Analyst 1999 (Issue No. 27, June 1999) page 2) and Chimei-Asahi Corporation (Taiwan) (S. Fukuoka et. al., Green Chem. Vol. 5 (year 2003) page 497). However, with these commercial catalysts, the reaction had to be carried out at high temperatures/pressures (30-80 bar) for high carbamate yields.
Srivastava et al (Catal. Lett. Vol. 89 (Year 2003) Page 81; Catal. Lett. Vol. 91 (Year 2003) Page 133) reported the use of metal phthalocyanines encapsulated in zeolite-Y and porous titanosilicate molecular sieves. US Pat application 20040242903 A1 reports the high performance zinc-substituted polyoxometalate solid catalysts. But in these applications an additional homogeneous Lewis base co-catalyst/promoter such as N,N-dimethyl aminopyridine is essential for high cyclic carbonate yields. This additional requirement of the homogeneous co-catalyst/promoter, hence, does not make the catalyst completely heterogeneous. Although the solid catalyst could be reused, in every recycle experiment the homogeneous, co-catalyst/promoter needs to be freshly added. It is, therefore, highly desirable to have a process for cyclic carbonate wherein the homogeneous co-catalyst/promoter can be completely avoided and the reaction occurs “truly” on the heterogeneous catalyst phase.
The present invention relates to an improved process for production of cyclic carbonates from epoxides using an adenine based completely “heterogeneous”, ordered, mesoporous, bifunctional, organo-inorganic, silica-based catalyst. The solid catalyst of the present invention is a modified, mesoporous, ordered silica with a Lewis acid metal ion (preferably tetrahedral Ti4+ ions by grafting) as well as with an organic base (preferably adenine or amine by anchoring). The catalyst is more efficient exhibiting synergism when both these constituents are present together on the mesoporous silica surface. The catalyst could be separated easily by centrifugation or by simple filtration and reused in several recycling experiments. No additional co-catalysts/promoters (unlike in the prior art catalysts) are required. Most importantly, the catalyst is highly efficient and only a small amount is needed to carryout the reaction. The process is atom-efficient and the reaction conditions like temperature and pressure are only moderate. Co-existence of dispersed, tetrahedrally coordinated Ti sites and heterogenized adenine/amine molecules and their synergism are the unique features of the catalyst of the present invention that makes this system more efficient for the cycloaddition reaction by activating the epoxide and CO2 molecules, simultaneously.
The bifunctional nature of the catalyst system with these particular active sites combination facilitates the availability of more amounts of activated CO2 and epoxides for the cycloaddition reaction to occur.